1. Field of the Invention
The present invention relates to a tubular structure for fixing a particulate matter detection device. More particularly, it relates to a tubular structure for fixing particulate matter detection device which is excellent in heat release properties.
2. Description of the Related Art
A flue exhaust gas or a diesel engine exhaust gas includes particulate matter (PM) such as soot, and has been a cause for air pollution. To remove this particulate matter, a filter (a diesel particulate filter: DPF) made of a ceramic material or the like is broadly used. The ceramic DPF can be used for a long period of time, but a defect such as cracking or melting sometimes occurs owing to heat deterioration or the like, and the particulate matter might leak, though its amount is small. When such a defect occurs, it is remarkably important from the viewpoint of the prevention of the air pollution to immediately detect the occurrence of the defect and to recognize the abnormality of the device. As a method of detecting the occurrence of such a defect, a method is suggested in which a particulate matter (PM) detection device is disposed on the downstream side of the DPF (e.g., see Patent Document 1).
Moreover, it is similarly important to detect nitrogen oxide (NOx) from the viewpoint of the prevention of the air pollution, and a nitrogen oxide detection device is suggested (e.g., see Patent Document 2).    [Patent Document 1] JP-A-60-123761    [Patent Document 2] JP-A-02-238354
The above particulate matter detection device or nitrogen oxide detection device is usually attached to an exhaust gas passing pipe or the like, to detect the particulate matter or nitrogen oxide included in the exhaust gas which circulates through the pipe. However, if this device is attached to, for example, a pipe for discharging the exhaust gas from a car engine, a stone struck up by a moving car or the like might collide with the device to break down the device. To solve the problem, it has also been suggested that, for example, the outside of the detection device be covered with a double tube and fixed to the pipe.
However, when the outside of the particulate matter detection device is covered with the double tube, for example, a problem occurs that the inside of the device has a high temperature, whereby a measured value becomes inaccurate.
The present invention has been developed in view of such problems of the conventional technology, and an object thereof is to provide a tubular structure for fixing particulate matter detection device which is excellent in heat release properties.